The present invention relates to a matrix display device, and more particularly to a liquid crystal, EL or ECD active matrix display device which uses a thin film transistor (TFT).
The active matrix display which uses the TFTs can offer a display device having a display unit and a peripheral drive circuits by TFT devices integrated on one substrate. This allows size reduction and cost reduction of the display device. The peripheral drive circuit was proposed in the Proceeding of IEEE, 59, 1566 (1971) and also proposed in JP-A-56-99396 and JP-A-57-201295.
Those circuits can drive display elements such as liquid crystal, EL and ECD by a smaller number of switching elements such as TFT's, and reduce the number of external connections, but still have rooms for improvement. Firstly, a signal voltage applied to a display element is held by a signal line capacitance C1 when a switching element such as TFT of the drive circuit is turned off, and applied to a switching element such as TFT of a picture element whose scan voltage of the display element has been selected. The voltage applied to a liquid crystal layer is determined by a capacitance proportion of the signal line capacitance C1 (if a capacity is created as required, it is paralelly added) and a capacitance C1.sub.c of the liquid crystal layer. Thus, the signal line capacitance C1 is selected to be sufficiently larger than the capacitance C1.sub.c of the liquid crystal layer. If a resistance R.sub.c between a signal electrode and a scan electrode which crosses thereto in a two-layer overcross structure is small, or if a resistance between a gate electrode of the switching element such as TFT and a drain electrode is small, the voltage held in the signal line capacitance C1 is discharged through the resistor and the voltage applied to the switching element such as TFT of the display unit is lowered. Such phenomenon takes place when any one of the switching elements such as TFT's or two-layer crossover connected to the signal line has an insufficient resistance. With such signal wiring, the voltage applied to the switching element such as TFT of the display unit is lowered. As a result, a display pattern of dark line is fixed and ununiformity in the display image takes place. In a worst case, a line defect takes place.
Secondly, input data is serially supplied from a video signal and the voltage applied to the display unit is driven by a point sequential scan operation or a time division sequential scan with a plurality of wires being in one block. As a result, there is a period in which no voltage is applied to the signal electrode, and there is a picture cell to which the voltage is applied for a short period. If a mutual conductance gm of the switching element such as TFT of the display unit is sufficiently high, the switching element such as TFT can fully charge the display element layer such as liquid crystal in a short period. However, if the mutual conductance gm is not high, the voltage is not applied to the display element layer such as liquid crystal for the picture cell whose voltage application period is short. As a result, ununiformity of display takes place or the number of scan lines of the display unit is limited by the limitation of the voltage application period.
The prior art device thus has problems in the drive characteristic of the display unit, the uniformity of the displayed image, the characteristic of the switching element such as TFT of the display unit and the need for formation of a high insulation film of a control electrode of the switching element such as TFT over the entire display area.
By using a line at-a-time scan circuit like a liquid crystal driving LSI, the above problems may be solved. However, since the circuits in the LSI require high speed transistor elements, the TFT element which uses the non-crystalline (for example, amorphous or polycrystalline) semiconductor film is not sufficient in its operation speed. Further, the LSI needs a complex circuit configuration and uses a number of transistor elements per stages. As a result, a yield of the circuit is low in a large size display.